The molecular weight of bovine plasma albumin (BPA) was examined by ultracentrifugal molecular weight determinations at low pH values in various concentrations of guanidinium thiocyanate (GSCN). The electrostatic charge on BPA under such conditions is approximately +100 for a molecular weight of about 70,000. It was shown that the value of Bwas obtained from the equation (1/Mwapp) = (1/Mwext) + Bw(Ca+ Cb)/2 goes through a minimum at concentrations slightly less than 2.5 M GSCN for both dialyzed and undialyzed samples. The results illustrate that the value of the net electrostatic charge on BPA varies with the absolute concentration of salt and BPA. The minimum Bwis due to a reversal of charge phenomenon. At 2.5 M GSCN or higher the net charge on BPA is negative rather than positive because of the association of counterions. The greater the solubility of the counterion, the greater will be the reversal of charge effect. Addition of a more soluble salt therefore reduces the value of Bwfaster than a less soluble salt. The extrapolated molecular weight values of BPA were the